1. Field of the Invention
The present invention relates to a method of surface pre-treatment of a variety of ink-receiving substrates of different types, wherein treatment energy is applied to the surface of the substrates in a controlled atmosphere that contains nitrogen and oxygen, and the amount of energy per surface area is adjusted dependent upon the type of substrate.
2. Description of Background Art
When liquid ink is to be applied to the surface of the substrate, e.g. in an ink jet printer, it is frequently desired or necessary to pre-treat the substrate in order to increase the surface energy of the substrate to such a level that the surface can be wetted with the liquid ink. The pre-treatment may for example be a plasma treatment in which a plasma jet is directed onto the surface of the substrate so that ions contained in the plasma will react with the substrate surface. In another embodiment, the pre-treatment may include a corona discharge. In any case, the pre-treatment includes a transfer of energy to the surface of the substrate in order to induce a reaction that modifies the chemical and/or physical properties of the substrate surface.
U.S. Pat. No. 7,150,901 discloses a method of the type indicated above, wherein a plasma treatment is performed in the presence of ambient air or in a pure nitrogen atmosphere.
JP 6 041 337 A discloses a method for controlling the surface energy of a plastic substrate by means of a plasma or corona treatment with a mixed gas containing a fluorine-containing compound, while adjusting the mixing ratio of the components of the mixed gas.
When an ink droplet is applied to the surface of the substrate, the size of the resulting ink dot will depend upon the speed with which the liquid ink spreads over the surface of the substrate, in relation to the speed with which the ink dries-out by evaporation of the solvent, and in relation to the speed with which the liquid is absorbed into the substrate.
The surface treatment may have an influence on the absorption speed, especially in case of porous substrates, but will mainly have an effect on the speed with which the liquid spreads, because the contact angle which the liquid/air meniscus of the droplet forms with the surface of the substrate will be dependent upon an equilibrium between the surface tensions of the liquid-to-air surface, the liquid-to-substrate surface and the substrate-to-air surface. In general, the spreading speed of the liquid and, consequently, the dot size will increase when the intensity of the pre-treatment is increased. Thus, the pre-treatment provides a possibility to control the dot size of the ink dots on the substrate.